There are various converters hat change one magnitude DC voltage to another. Conventional converters such as forward and flyback converters are well described as the prior art. Many text's such as George Chryssis "High-Frequency Switching Power Supplies: Theory and Design", McGraw Hill Book Co., can explain the operation of converters such as these.
Conventional converters all use forced commutation means to control the voltage and current in the regulation and power transfer process. This forced commutation means of regulation causes two types of problems. The first is the losses associated with the forced commutation. Since there is a finite time associated with the turn on and turn off of a switching element, the current flowing in the switch and the voltage across the switch will overlap during switching transitions causing a loss called switching loss. The faster the switching transition, the lower the switching loss. The second type of problem is the noise generated by the forced commutation process. This noise is predominantly caused by the rate of change of voltage, dV/dT, on the high voltage switch. This change in voltage, coupled with the parasitic capacitance of the transformer primary-to-secondary windings, causes common mode current to flow through this path. In order to help control this noise at turn off, a load line snubber is typically used. This approach requires board real estate to implement, as well as wastes energy and cost money. Furthermore, this approach only helps at turn off and the noise generation problem still exists at turn on. Ultimately, the efficiency problem associated with the conventional topology is the most severe problem. Current topology operates at about 73% efficiency, causing significant losses to be absorbed by the power supply package, thus driving the power supply density down.
Two types of newer generation prior art topology, shown in FIGS. 1A and 1B, attack the problems associated with the previous art described above. The first type of topology is the series resonant power supply switching at zero current. These supplies reduce the noise generated by switching at very close to zero current (magnetizing current) and eliminate about half of the switching losses without the need for the load line shaping circuits. There is still the loss associated with switching voltage at the switch turn on point however. During the switching transition, there can be significant voltage across the switch that essentially charges the parasitic capacitance of the switch, which is ultimately discharged by the action of turning on the switch. A typical power fet, having a drain-to-source capacitance of 120 pf, operating at 100 khz with a voltage of 700 volts across it can have a parasitic loss of approximately 2.98 watts. The rate of change of current is also lower as the current (dI/dT) is sinusoidal. The major problem with the type of converter shown in FIG. 1A is in the RMS current relationship vs line. The peak currents flowing in the resonant switches, transformer, and output diodes are at their lowest during low line operation when the duty ratio is maximum. The efficiency at this point can be in the low eighties. When the input voltage is raised, however, the RMS current flowing in the power supply elements will rapidly increase causing the RMS current to increase by as much as 1.7. This ultimately causes the efficiency to drop into the low 70's, where the original forced commutation power supply operates. There are ways to overcome this phenomenon, however, but this would require the addition of a new converter in front of the resonant to stabilize the input voltage, thus optimizing the efficiency. The extra cost incurred and extra board real estate required makes this approach viable only in the higher power arena. Operation of this type of converter is exemplified in U.S. Pat. No. 4,415,959 to Vinciarelli.
A more recent approach to this problem with RMS currents is shown in FIG. 1B and exemplified in Matsushita Japanese patent No. 1503925. In this type of resonant converter the resonant circuit is a combination of both series and parallel resonant circuits. In this type of approach the frequency shift of the power supply over the regulating area is greatly reduced. In the Vinciarelli type converter the frequency shift could be over 10:1 for all conditions, and even more if no-load operation is required. In the series/parallel converter, the frequency shift is a function of the ratio between the parallel inductance and series inductance of the tank circuit. Practical frequency shifts can thus be enjoyed of only 2:1. Since the energy being transferred to the output load is a function of not only the voltage in the resonant circuit (1/2CV2.sup.2 F) but is also a function of the phase relationship between two resonant circuits, the RMS current flowing in the switches, transformer, and diodes changes little with corresponding changes in input line. This, in effect, stabilizes the efficiency vs line characteristics of the power supply. The resonant tank losses tend to increase with line in the series parallel converter but the RMS currents in the switch, transformer, and magnetic tend to remain the same. One of the major drawbacks of the Matsushita approach is the inability to adjust the tank operating voltage. In all resonant converters, the Q of the tank circuit is of paramount importance to the overall efficiency of the power converter. One dominant means of controlling losses for any given power output in the resonant design process is to pick a low operating current in the tank circuit. In previous prior art converters (Vinciarelli: U.S. Pat. No. 4,415,959, Japanese Patent No. 1503925) the operating tank voltage is not adjustable independent of the operating voltage ratios of the converter. This forces the designer to adjust other equally important parameters such as leakage inductance, resonant capacitor value, and operating flux density. The result of this is a greatly reduced design efficiency due to lower Q as well as a larger tank capacitor and potentially a more difficult magnetic element from a manufacturing viewpoint. Also, since the ratio of tank magnetizing inductance vs tank voltage cannot be manipulated in prior art converters, the minimum frequency shift is difficult to optimize. In the prior art converter by Archer U.S. Pat. No. 4,774,649, a novel resonant converter is described which is constructed on a integrated magnetic element. In this approach, some control over the tank operating voltage is available; however, this control is given at the expense of other variables and the integrated magnetic transformer tends to have a low operating Q for this reason.
Finally, the remaining problem associated with both the Vanciarelli and Matsushita converters is the inability to operate in both the zero current and zero voltage mode simultaneously. As stated earlier, a controlled rate of change of voltage (dV/dT) is desirable to achieve quiet operation from an EMI/RFI standpoint. To further complicate this requirement, it is desirable to achieve this without the use of external components, as part count and size have a direct impact on the size of the finished supply.